Assume FS_IOC_GETFLAGS may trash output on error

Don't assert that the attr value is reasonable (<= UINT_MAX) when
FS_IOC_GETFLAGS returns an error, because it may change the the value
to something invalid in that situation.

This has been observed to occur when, for example, the underlying
filesystem does not support the operation,

Signed-off-by: Mark J Hewitt <m.hewitt@computer.org>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>